Streptococcus pneumoniae, commonly referred to as the pneumococcus, is an important pathogenic organism. The continuing significance of Streptococcus pneumoniae infections in relation to human disease in developing and developed countries has been authoritatively reviewed (Fiber, G. R., Science, 265:1385-1387 (1994)). That indicates that on a global scale this organism is believed to be the most common bacterial cause of acute respiratory infections, and is estimated to result in one million childhood deaths each year, mostly in developing countries (Stansfield, S. K., Pediatr. Infect. Dis., 6:622 (1987)). In the USA it has been suggested (Breiman et al, Arch. Intern. Med., 150:1401 (1990)) that the pneumococcus is still the most common cause of bacterial pneumonia, and that disease rates are particularly high in young children, in the elderly, and in patients with predisposing conditions such as asplenia, heart, lung and kidney disease, diabetes, alcoholism, or with immunosupressive disorders, especially AIDS. These groups are at higher risk of pneumococcal septicaemia and hence meningitis and therefore have a greater risk of dying from pneumococcal infection. Over 50,000 cases of invasive pneumococcal disease (meningitis and bacteraemia) are believed to occur annually in the United States. The pneumococcus is also the leading cause of otitis media and sinusitis, which remain prevalent infections in children in developed countries, and which incur substantial costs. S. pneumoniae is responsible for approximately seven million cases of middle ear infections in children under two years of age in the United States alone.
The need for effective preventative strategies against pneumococcal infection is highlighted by the recent emergence of penicillin-resistant pneumococci. It has been reported that 6.6% of pneumoccal isolates in 13 US hospitals in 12 states were found to be resistant to penicillin and some isolates were also resistant to other antibiotics including third generation cyclosporins (Schappert, S. M., Vital and Health Statistics of the Centres for Disease Control/National Centre for Health Statistics, 214:1 (1992)). The rates of penicillin resistance can be higher (up to 20%) in some hospitals (Breiman et al, J. Am. Med. Assoc., 271:1831 (1994)). Since the development of penicillin resistance among pneumococci is both recent and sudden, coming after decades during which penicillin remained an effective treatment, these findings are regarded as alarming.
For the reasons given above, there are therefore compelling grounds for considering improvements in the means of preventing, controlling, diagnosing or treating pneumococcal diseases.
Various approaches have been taken in order to provide vaccines for the prevention of pneumococcal infections. Difficulties arise for instance in view of the variety of serotypes (at least 90) based on the structure of the polysaccharide capsule surrounding the organism. Vaccines against individual serotypes are not effective against other serotypes and this means that vaccines must include polysaccharide antigens from a whole range of serotypes in order to be effective in a majority of cases. An additional problem arises because it has been found that the capsular polysaccharides (each of which determines the serotype and is the major protective antigen) when purified and used as a vaccine do not reliably induce protective antibody responses in children under two years of age, the age group which suffers the highest incidence of invasive pneumococcal infection and meningitis.
A modification of the approach using capsule antigens relies on conjugating the polysaccharide to a protein in order to derive an enhanced immune response, particularly by giving the response T-cell dependent characteristics. This approach has been used in the development of a vaccine against Haemophilus influenzae, for instance. There are, however, issues of cost concerning both the multi-polysaccharide vaccines and those based on conjugates. In addition, the composition of the conjugate vaccines preferably requires to be varied to accommodate different geographical and demographical populations as the serotype coverage that they offer is limited. There may also be problems with conjugate carrier-induced suppression or overload due to the relatively large total dose of carrier protein administered.
A third approach is to look for other antigenic components which offer the potential to be vaccine candidates. This is the basis of the present invention. Using a specially developed bacterial expression system, we have been able to identify a group of protein antigens from pneomococcus which are associated with the bacterial envelope or which are secreted.